1. Field of the Invention
The present invention relates to an electronic flash camera having a fixed focus lens and a scanning aperture, in general, and to such a camera wherein an exposure control system thereof increases the lens's depth of field for relatively close subjects to improve the sharpness of an image of such subjects formed by the lens, in particular.
2. Description of the Prior Art
It is well known that the sharpness of an image formed by a lens at an image plane is primarily dependent upon the exactness of focus of the image at the image plane. When a lens is focused to produce a sharp image of a particular object at the image plane, other objects that are closer or further away do not appear equally sharp. The decline in sharpness is gradual and there is a spacial zone extending in front of and behind the focused subject where the image misfocus is too small to be easily noticeable and therefore can be accepted as sharp. This zone is commonly referred to as the depth of field of the lens.
In the beginning, photographic cameras only employed lenses of the fixed focus type. In order to form photographic images of acceptable sharpness with such lenses over the greatest possible range of subject distances, their optical characteristics had to be chosen such that the near distance of their depth of field could only extend to within approximately 4 to 5 feet of the camera for an f/14 lens, if images of distant objects (objects at infinity) were to have an acceptably sharp focus at the camera's film plane. An image of an object produced by this type of lens that is located closer to the camera than this near distance would appear blurred or noticeably out of focus.
The production of sharp images of distant as well as relatively close object is not a problem with an adjustable focus lens. With such a lens, the focus distance and its associated depth of field can be adjusted to produce acceptably sharp images of objects located at virtually any object distance. While an adjustable focus lens has many advantages, including the just-mentioned ability to vary its depth of field, such a lens has certain disadvantages. Among the disadvantages are cost and the increased susceptibility to mechanical failure of its focusing mechanism over that of a fixed focus lens.
An electronic flash, fixed focus lens camera that is capable of forming sharp images of objects located closer to the lens than the near distance of its normal depth of field, has been disclosed. In U.S. Pat. No. 4,459,005 to Harvey, for example, exposure control apparatus for an electronic flash, fixed focus lens camera includes means for controlling exposure with either of two predetermined apertures. The apparatus includes proximity sensing means for detecting when a subject to be photographed is within a given near distance of the camera and a mechanism for restricting the exposure aperture to a size that is less than the aperture size the exposure control mechanism would normally employ when the subject is within the aforementioned given near distance. The proximity sensing means includes a light emitting diode for illuminating a subject to be photographic with infrared (IR) light and a sensor for measuring subject IR reflectivity. Among the disadvantages inherent in this type of electronic flash, exposure control apparatus are the relatively complex mechanism required to establish the selectable above-noted predetermined apertures, the relatively large amount of time required to actuate the aperture size controlling mechanism to its aperture size determining position prior to initiating an exposure interval and the inability to easily change the size of the above-noted predetermined aperture once they have been incorporated into the exposure control system design of a camera.
In U.S. Pat. No. 4,894,678 to Farrington et al., an exposure control system is disclosed that is also capable of forming sharp images of objects located closer to the lens than the near distance of its normal depth of field. An image of a subject in a scene formed by a fixed focus lens through a scanning aperture of an electronic flash camera, particularly that of a subject located closer to the lens than the near distance of the lens's normal depth of field, has its clarity or sharpness substantially improved by firing the electronic flash during exposure at the smallest possible aperture for optimum subject-image sharpness and overall scene exposure. The appropriate aperture employed to produce such an image is a function of ambient scene brightness and of the infrared light from the electronic flash reflected from a subject within the scene which are sensed prior to an exposure interval. While this type of exposure control system is very effective in producing photographs having optimum subject-image sharpness and overall scene exposure, it is a relatively costly method for controlling the exposure of a photographic image in a camera.